The present invention relates to the construction of improved electrolytic cells useful as units of a filter press cell arrangement. The present cells are particularly useful in the electrolysis of alkali metal chlorides, such as sodium chloride, to produce alkali metal hydroxides, such as sodium hydroxide, together with chlorine and hydrogen.
A filter press arrangement typically consists of a plurality of separate cell units having planar electrode elements generally mounted in a vertical position separated along their active faces by a barrier, such as a diaphragm or membrane layer. The filter press cell units may be monopolar or bipolar and may be appropriately connected in series or parallel to form a circuit or cell bank.
Chlorine and alkali metal hydroxides are essential and large volume commodities as basic industrial chemicals. Plants producing 500 to 1000 tons of chlorine per day are not uncommon. Such plants typically utilize a large number of individual electrolytic cells having current capacities of several hundred thousand amperes. Thus, minor improvements in individual cell operation or performance have major economic benefits because of the volume of the products produced.
Upon the application of direct, electrolyzing current to an electrolytic cell containing an aqueous solution of an alkali metal chloride as the electrolyte, hydrogen and alkali metal hydroxide are produced at the cathode and chlorine is produced at the anode.
Electrolytic cells that are commonly employed commercially for the conversion of alkali metal halides into alkali metal hydroxides and halides may be considered to fall into the following general types: (1) diaphragm, (2) mercury and (3) membrane cells.
Diaphragm cells utilize one or more diaphragms permeable to the flow of electrolyte solution but impervious to the flow of gas bubbles. The diaphragm separates the cell into two or more compartments. Although diaphragm cells achieve relatively high product per unit floor space, at low energy requirements and at generally high current efficiency, the alkali metal hydroxide product, or cell liquor, must be concentrated and purified. Such concentration and purification is usually accomplished by a subsequent evaporation step.
Mercury cells typically utilize a moving or flowing bed of mercury as the cathode and produce an alkali metal amalgam in the mercury cathode. Halide gas is produced at the anode. The amalgam is withdrawn from the cell and treated with water to produce a high purity alkali metal hydroxide.
Membrane cells utilize one or more membranes or barriers separating the catholyte and the anolyte compartments. The membranes are permselective, that is, they are selectively permeable to either anions and cations. Generally, the permselective membranes utilized are cationically permselective. Usually, the catholyte product of the membrane cell is a relatively high purity alkali metal hydroxide ranging in concentration from about 250 to about 350 grams per liter.
The advent of dimensionally stable anodes has permitted even narrowing of the space, or gap, between the electrodes of a cell, thereby facilitating progressively higher cell efficiency. In the operation of circuits or banks of electrolytic cells, it is advantageous to have the electrode gap uniform in order that the circuit be balanced.
Circuits or banks of filter press cells are formed by the assembly of individual cell components. For example, in the case of a monopolar arrangement, the components typically would comprise a plurality of anodes mounted in anode frames and cathodes mounted in cathode frames. The anodes and cathodes are separated along their active faces by a permeable barrier, such as a diaphragm or membrane, and along the inner periphery of the frames by a pliable or elastic gasket member. The assembly is completed by coupling or pressing the components together, hydraulically or by means of threaded connectors, to compress the gasket members to form gas and liquid-tight seals between the individual units. Because of the differences in gasket materials and the required compression sufficient to obtain a gas and liquid-tight seal, it has heretofore been a difficult task to obtain and to maintain a desired electrode gap in a filter press arrangement.